B stands for bombs; the bombs are much bigger.
So, brother, do not be too fast on the trigger.

F stands for fission; that is what things do
When they get wobbly and big and must split in two.
And just to confound the atomic confusion
What fission has done may be undone by fusion.

H has become a most ominous letter;
It means something bigger, if not something better.

S stands for secret; you can keep it forever —
Provided there’s no one abroad who is clever.